1. Field of the Invention
The present invention relates to a method of producing a metal closed-section member formed in a cross-sectionally closed shape, from a metal plate workpiece.
2. Description of the Related Art
As is commonly known, in view of ensuring strength and rigidity of a vehicle body to provide enhanced safety in a vehicle, such as an automobile, it is necessary for a vehicle body member making up a part of a basic structure of the vehicle body, such as a pillar member, a front or rear side frame, or a dash cross-member located in a lower region of a front surface of a dash panel, to achieve higher strength and rigidity, as well as weight reduction for improvement in fuel economy.
With a view to meeting the above needs, the vehicle body member is comprised of a metal closed-section member, in many cases. As one example of means for producing such a metal closed-section member, there has been commonly known a method which comprises subjecting each of a pair of metal plate workpieces, such as steel plates, to press working to form a pair of cross-sectionally hat-shaped (or cross-sectionally angular C-shaped) plate workpieces each integrally having a flange portion extending outwardly from a respective one of two free edges thereof, butting respective flange portions of the pair of cross-sectionally hat-shaped plate workpieces in such a manner as to define a closed-section space therebetween, and then welding the flange portions together to produce a metal closed-section member.
There has also been known a method of producing a triangular-shaped tube as a metal closed-section member, wherein the method comprises a first bending step of bending opposite ends of a steel plate pre-cut into a given shape, to have a given curvature, a second bending step of pressing and bending a central portion of the steel plate bent in the first bending step, a restraint-forming step of pressing the opposite ends of the steel plate bent in the second bending step, downwardly from thereabove, while applying restraint forces to the central portion of the steel plate from respective opposed rightward and leftward directions perpendicular to a longitudinal direction of the steel plate, so that the steel plate is formed under restraint into a given shape where the opposite ends are butted together, and a welding step of, after the restraint-forming step, welding the butted portions of the opposite ends without releasing the restraint forces, as disclosed, for example, in the Japanese Patent Laid-Open Publication No. 2000-51932.
Late years, in vehicles, such as automobiles, there has been an increasing need for further weight reduction to improve fuel economy, and vehicle body members occupying a relatively large part of vehicle weight have also been required to accelerate further weight reduction. Therefore, in regard to a vehicle body member to be formed as a metal closed-section member, it is desired to produce the metal closed-section member without providing a flange portion thereto.
The method disclosed in the Japanese Patent Laid-Open Publication No. 2000-51932 can produce a triangular closed-section shaped tube without providing a flange portion thereto. However, a vehicle body member for use in a vehicle, such as an automobile, generally has a complicated shape, wherein a cross-sectional shape is highly likely to change complicatedly in a longitudinal direction of the member. Thus, in case where the production method disclosed in the Japanese Patent Laid-Open Publication No. 2000-51932 is applied to a vehicle body member, it is necessary to prepare as a forming die a plurality of plates each formed with a cutout having the same shape as that of a cross section of a metal closed-section member to be formed. Moreover, during the step of pressing opposite ends of the bent steel plate downwardly from thereabove while applying restraint forces to a central portion of the steel plate in respective opposed rightward and leftward directions perpendicular to a longitudinal direction of the steel plate, so that the steel plate is formed under restraint into a given shape by the plurality of plates each formed with the cutout having the same shape as that of a cross section of a closed-section member to be formed, a forming defect, such as distortion or deformation, is liable to occur, which is likely to cause considerable difficulty in producing the metal closed-section member with a high degree of accuracy.